Can you verify your Licensing model?
>

Are you using Premium licenses, or Per Flow/Per App licenses? It's possible they are all used up so others cannot see it, or they are not properly added to the environment with permissions and roles.
> Which environment? I got that one:

Are these part of a Solution Or simply standalone flows?
> Standalone
Can you please also, go to the Power Apps or Power Automate Home Page (or ask IT to do it)
Go to the admin
Go to the Environment of the Flows
Go to All Users
Find the user(s) one of them that cannot see the flows.
Run a diagnostic on them from the Top menu (after selecting them in the pre step)
It will give us more information
> I will try to get this information by the it - it is a institution with 40'000 employees ...
Also, its possible its just going to take time, so what you can try it this
1. You go to the flow
2. Click on the Details Page
3. Grab the Link once it opens
4. Share with the person who cannot see it
5. See if they open the Details page using the Direct link to the Details page.
If so, then its just a refresh/timing issue
If NOT it should also give us more information (Permissions etc)
Cheers
If any of these help you resolve the issue Please Mark my answer as such and maybe a like.
Thanks